Method for producing a hot strip of a bainitic multi-phase steel having a Zn—Mg—Al coating, and a corresponding hot strip

Abstract

The invention relates to a method for producing a hot-rolled strip composed of a bainitic multi-phase steel and having a Zn—Mg—Al coating, comprising the following steps: melting a steel melt containing (in weight percent): C: 0.04-0.11, Si: <=0.7, Mn: 1.4-2.2, Mo: 0.05-0.5, Al: 0.015-0.1, P: up to 0.02, S: up to 0.01, B: up to 0.006, and at least one element from the group Nb, V, Ti in accordance with the following condition: 0.02<=Nb+V+Ti<=0.20, the remainder being iron including unavoidable steel-accompanying elements resulting from the melting process, casting the steel melt into a preliminary material, in particular a slab or a block or a thin slab, hot rolling the preliminary material into a hot-rolled strip having a final rolling temperature in the range of 800 to 950° C., cooling the hot-rolled strip to a winding temperature less than 650° C., winding the hot-rolled strip at a winding temperature less than 650° C., cooling the wound hot-rolled strip to room temperature in still air, wherein the microstructure of the wound hot-rolled strip then has a bainite fraction greater than 50% after the hot rolling, heating the hot-rolled strip to a temperature greater than 650° C. and less than Ac3, in particular less than Ac1+50° C., cooling the hot-rolled strip to zinc bath temperature, hot-dip coating the heated hot-rolled strip in a zinc alloy molten bath containing (in weight percent): Al: 1.0-2.0, Mg: 1.0-2.0, the remainder being zinc and unavoidable impurities. The invention further relates to the hot-rolled strip produced in accordance with the method above and to shaped, dynamically highly loadable components, in particular motor vehicle parts, that are produced from said hot-roiled strip and that are resistant to corrosive and abrasive influences.

Claims

1. A method for producing a hot strip of bainitic multi-phase steel having a Zn—Mg—Al coating, said method comprising: smelting a steel melt consisting of, in weight percent, C: 0.04-0.11 Si: ≤0.7 Mn: 1.4-2.2 Mo: 0.05-0.5 Al: 0.015-0.1 P: up to 0.02 S: up to 0.01 B up to 0.006 and at least one element from the group Nb, V, and Ti according to the following condition:
0.02≤Nb+V+Ti≤0.20 with a remainder being iron and unavoidable impurities; casting the steel melt to form a precursor material; hot rolling the precursor material with an end rolling temperature in a range of 800 to 950° C. to form a hot strip; cooling the hot strip to a reeling temperature of less than 650° C.; reeling the hot strip at a reeling temperature of less than 650° C.; cooling the reeled hot strip to room temperature in stationary air, with the reeled hot strip having a microstructure with a bainite content of greater than 50% after hot rolling; heating the hot strip to a temperature of greater than 650° C. and less than Ac3; cooling the hot strip to a zinc bath temperature; hot-dip coating the hot strip in a zinc alloy melt bath containing, in weight percent, Al: 1.0-2.0 Mg: 1.0-2.0 with the remainder being zinc and unavoidable impurities, wherein the magnesium content in the zinc alloy melt bath is less than the aluminium content, such that the hot-dip coating of the heated hot strip in the zinc alloy melt bath results in a two-stage solidification of the melt in which in a first solidification stage, zinc primary crystals are produced and in a second solidification stage, a fine-grained ternary Zn—Al—Mg eutectic is formed.

2. The method of claim 1, wherein the precursor material is a slab or a block or a thin slab.

3. The method of claim 1, wherein the hot strip is heated to a temperature of greater than 650° C. and less than Ac1+50° C.

4. The method of claim 1, wherein annealing and heating the hot strip at a temperature of greater than 650° C. and less than Ac3 take place in one working step and the hot strip is hot-dip coated immediately after the heating and cooling to zinc bath temperature.

5. The method of claim 4, wherein the temperature of annealing and heating the hot strip is greater than 650° C. and less than Ac1+50° C.

6. The method of claim 1, wherein the hot strip is hot-dip coated in a zinc alloy melt bath at a temperature of 405 to 470° C.

7. The method of claim 6, wherein the bath temperature is of 410 to 430° C.

8. The method of claim 1, wherein the steel melt has a C content of 0.06 to 0.10 weight percent, an Si content of 0.05 to 0.50 weight percent, and a total of the contents of Nb+Ti is in a range of 0.05 to 0.20.

9. The method of claim 1, wherein the content of each of the alloying elements from the group Nb, V, and Ti is at least 0.005 weight percent and less than or equal to 0.20 weight percent.

10. The method of claim 1, wherein a sum of the Ti and Mo contents is >0.1 weight percent and 0.7 weight percent.

11. The method of claim 1, wherein the hot-dip coated hot strip has a tensile strength Rm of 780 to 980 MPa.

12. The method of claim 1, wherein the hot-dip coated hot strip has a yield strength ReH of at least 680 MPa.

13. The method of claim 1, wherein the hot-dip coated hot strip has an elongation at fracture A pursuant to DIN EN ISO 6892-1:2009 of at least 10%.

14. The method of claim 1, wherein the zinc alloy melt bath has the magnesium content in a range of 1.0 to 1.2 weight percent and the aluminium content in the range of 1.3 to 1.7 weight percent.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) None

DESCRIPTION OF PREFERRED EMBODIMENTS

(2) None